The present invention relates to a pull-down circuit for a TTL compatible data output buffer using NMOS devices. Specifically, the circuit results in improvement of the under-shoot and over-shoot of the output node during load pull-down and in reduction of the bouncing of the on-chip ground node making the circuit particularly suitable for interfacing between 3 volt and 5 volt off-chip bus systems.
As CMOS technology improves, the need for interfacing between 3 volt and 5 volt systems increases. An off-chip driver (OCD) which provides the interface between a 3 volt chip and a 5 volt off-chip bus is a challenge to design because an on-chip pull-down circuit operating at 3 volts (2.8 volt in a worst case) must be able to discharge into a capacitive load charged to 5.6 volts to ground potential. When contrasted with a 5 volt chip discharging a 5 volt off-chip bus, the pull-down device on a 3 volt chip has to be able to operate with a minimum power supply voltage of 2.8 volts, which reduces the over-drive of the pull-down device. Consequently, the pull-down device for a 3 volt chip is larger than its 5 volt counterpart and more prone to under-shooting and over-shooting as well as larger ground bounce.
U.S. Pat. No. 5,144,165 entitled "CMOS Off-Chip Driver Circuits", assigned to the same assignee as the present invention, discloses a CMOS off-chip driver circuit which interfaces between 3 volts and 5 volts but the pull-down circuit is a common-source single stage NMOS circuit. The present invention concerns a different NMOS pull-down circuit which overcomes some of the limitations associated with common-source NMOS pull-down circuits as will be described below.
U.S. Pat. No. 4,430,582 entitled "Fast CMOS Buffer for TTL Input Levels" discloses a CMOS buffer (or voltage regulator) which controls the power supply voltage to the CMOS gates so that the CMOS gates maintain compatibility with the TTL interface levels with power supply and process variations.
U.S. Pat. No. 4,307,308 entitled "Digital Signal Conversion Circuit" discloses a level shifter which converts the TTL voltage levels to the operating range of the MOS logic circuits so that the two different circuits are able to communicate with one another.
Research Disclosure No. 27711, dated May, 1987, entitled "Three to Five Volt Off Chip Driver Interface Circuit" discloses an off-chip driver whose pull-down circuit comprises a common-source NMOS circuit in series with a zero-volt NMOS device. This circuit requires a zero-volt NMOS device for proper operation which is a drawback for most CMOS technologies since zero-volt NMOS devices are no longer used in such technologies.